We report extremely fast tandem ring-opening/ring-closing metathesis polymerization of a monomer containing two rather unreactive functional groups: cyclohexene and a terminal alkyne. When a third-generation Grubbs catalyst was used at low temperature, this tandem polymerization produced polymers with controlled molecular weights and narrow polydispersity indices. To explain this extremely fast polymerization, its reaction mechanism was studied. This new type of controlled polymerization allowed for the preparation of block copolymers using other conventional living metathesis polymerizations. The diene on the backbone of the polymer was postfunctionalized by sequential Diels–Alder and aza-Diels–Alder reactions, which led to selective functionalization depending on the stereochemistry of the diene.